Many different types of interchangeable lens cameras are presently available. Different manufacturers of cameras have, in the past, used different mounting arrangements for the interchangeable lenses, usually to preserve the individuality of their cameras and lenses. As a result, if the owner of a camera body and a series of lenses which fit the body decides to obtain a new body with a different mount, the old lenses cannot be used with the new camera body.
In addition, the trend in cameras has been towards full automation of the picture taking process including modes of operation such as shutter priority automatic exposure, aperture priority automatic exposure, and programmed automatic exposure. In each of these automatic modes of operation, it is necessary for the camera body to automatically set the diaphragm or lens aperture either to the selected value in the case of aperture priority (and then set the shutter speed for proper exposure), or to automatically set the diaphragm to a computed value based on shutter speed in the case of shutter priority, or based on a computed value in the case of programmed automatic exposure. For such automatic operation, most automatic lenses are provided with a diaphragm lever which is operated by a diaphragm control lever on the camer body.
For certain types of photography, and with certain cameras, it is possible to manually set the diaphragm or aperture by rotating the diaphragm ring, and the camera automatically selects the shutter speed for proper automatic exposure. In order, however, for the camera to select the shutter speed, the camera must know the diaphragm or aperture setting. Most lenses have a diaphragm value control lever for transmitting this information to the camera via a diaphragm value lever on the camera body.
To provide a bright viewfinder image, many cameras normally maintain the diaphragm full open until the instant that the picture is taken, at which time the diaphragm is automatically stopped down to the selected or oomputed value.
A common feature of all automatic lenses is that the lens must transmit information to the camera to indicate the fully opened diaphragm value of the lens. Many cameras can also receive from the lens information to indicate the fully opened diaphragm value as well as the fully close diaphragm value of the lens. Some camera manufacturers have adopted a digital system to transmit such information,whereas other camera manufacturers have adopted an analog system for such information.
Such information about the fully opened diaphragm value of the lens is indispensable to automatic operation of the camera and lens. Without this information about the fully opened diaphragm value of the lens, underexposure could frequently occur if the camera, or the user, selected a diaphragm value lower than the capability of the lens.
While these various automatic features and controls have preserved the individuality of the camera manufacturer's products, they have caused severe production problems for interchangeable lens makers who market lenses for such cameras, at comparatively low prices.
The problems with production by the interchangeable lens makers results from the fact that the same construction of lens must be provided with various different lens mounts to correspond to the different manufacturers of cameras, and in the past, different fully opened diaphragm values information systems or fully opened diaphragm value and fully closed diaphragm value information systems required additional lenses.
These many different features of different camera manufacturers have also presented severe inventory and stocking problems in camera stores. As a result, such dealers tend to stock lenses only for the more popular cameras, and often do not stock lenses for less popular cameras because of the inventory required. As a result, sales of less popular cameras have decreased, and the gap between the popular and less popular cameras has become greater.
Recently, several camera manufacturers have adopted a common lens mount known as a "K-mount" (K-type bayonnet mount) which permits mounting the same lens on cameras of different manufacturers. Although it is possible to use an interchangeable lens on cameras with the same mount, using a lens made for one camera on another camera, often does not provide for automatic operation. When using such an interchangeable lens on a different camera, the lens is usable only in a "manual mode" in that it is necessary for the user to determine and set both the diaphragm and the shutter speed, or at the very least, to preset the diaphragm. It is practically impossible, when using a lens from one camera on a different manufacturer's camera, to attain shutter priority automatic exposure or program automatic exposure because of the different controls and information transfer arrangements of different cameras.